The effect of weak lensing is to cause a very small (less than a few percent) variation in the apparent shapes of background galaxies, as measured by their ellipticities. The effect can only be detected by averaging over many galaxies - Euclid will measure around one billion - and the final lensing signal needs to be free of systematic error to a level that is one thousand times smaller than the measurement noise for an individual galaxy. This accuracy has to be obtained from measurements of galaxies whose signal-to-noise ratio may be as low as 10, and to avoid systematic error the effects of convolution by the telescope's point spread function (PSF) and of instrumental shear introduced by telecope imaging distortion must be corrected to high accuracy.

Because Euclid is above the Earth's atmosphere, the imaging is not affected by atmospheric "seeing", and from its orbit at L2 the telescope should have a very stable PSF. Nonetheless, the PSF and galaxy ellipticity measurement must be made extremely carefully, and this process is a major challenge in both statistical methods of data analysis and in developing our understanding of the telescope and its imaging system.